This invention relates to ultrasound imaging systems and, more particularly, to increasing the (sub)harmonic-to-fundamental ratio and the (sub)harmonic-to-noise ratio of contrast agent-generated (sub)harmonic signals in medical ultrasound imaging.
Conventional ultrasound imaging systems comprise an array of ultrasonic transducer elements which are used to transmit an ultrasound beam and receive the reflected beam from the object being studied. Such scanning comprises a series of measurements in which the focused ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasonic wave is received, beamformed and processed for display. Typically, transmission and reception are focused in the same direction during each measurement to acquire data from a series of points along an acoustic beam or scan line. The receiver is dynamically focused at a succession of ranges along the scan line as the reflected ultrasonic waves are received.
One type of conventional system includes a transmit beamformer that supplies high-voltage transmit waveforms in a plurality of channels via transmit/receive switches to a transducer array. The transmit beamformer and the transducer array have a broadband response and are capable of transmitting the maximum allowable acoustic power densities for better signal-to-noise sensitivity. The transducer array generates an ultrasonic transmit beam in response to the transmit waveforms, and this transmit beam propagates outwardly through the subject being imaged. The transducer frequency response acts as a bandpass filter. Ultrasonic energy echoed by the subject is received by the transducer array and focused by a receive beamformer. The transducer and receive beamformer have a broadband response. The focused signal is then filtered to attenuate undesired frequencies.
For ultrasound imaging, the array comprises a multiplicity of transducer elements arranged in one or more rows and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements in a given row can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. For a steered array, by changing the time delays and amplitudes of the applied voltages, the beam with its focal point can be moved in a plane to scan the object. For a linear array, a focused beam directed normal to the array is scanned across the object by translating the aperture across the array from one firing to the next.
The same principles apply when the transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object under study. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting a separate time delay (and/or phase shift) and gain to the signal from each receiving transducer element.
An ultrasound image is composed of multiple image scan lines. A single scan line (or small localized group of scan lines) is acquired by transmitting focused ultrasound energy at a point in the region of interest, and receiving the reflected energy over time. The focused transmit energy is referred to as a transmit beam. During the time after transmit, one or more receive beamformers coherently sum the energy received by each channel, with dynamically changing phase rotation or time delays, to produce peak sensitivity along the desired scan lines at ranges proportional to the elapsed time. The resulting focused sensitivity pattern is referred to as a receive beam. Resolution of a scan line is a result of directivity of the associated transmit and receive beam pair.
The output signals of the beamformer channels are coherently summed to form a respective pixel intensity value for each sample volume in the object region or volume of interest. These pixel intensity values are log-compressed, scan-converted and then displayed as an image of the anatomy being scanned.
Conventional ultrasound transducers transmit a broadband signal centered at a fundamental frequency f0, which is applied separately to each transducer element making up the transmit aperture by a respective pulser. The pulsers are activated with time delays that produce the desired focusing of the transmit beam at a particular transmit focal position. As the transmit beam propagates through tissue, echoes are created when the ultrasound wave is scattered or reflected off of the boundaries between regions of different density. The transducer array is used to transduce these ultrasound echoes into electrical signals, which are processed to produce an image of the tissue. These ultrasound images are formed from a combination of fundamental (linear) and harmonic (nonlinear) signal components, the latter of which are generated in nonlinear media such as tissue or a blood stream containing contrast agents. With scattering of linear signals, the received signal is a time-shifted, amplitude-scaled version of the transmitted signal. This is not true for acoustic media which scatter nonlinear ultrasound waves.
The echoes from a high-level signal transmission will contain both linear and nonlinear signal components. In certain instances ultrasound images may be improved by suppressing the fundamental and emphasizing the harmonic (nonlinear) signal components. If the transmitted center frequency is f0, then tissue/ contrast nonlinearities will generate harmonics at kf0 and subharmonics at f0/ k, where k is an integer greater than or equal to 2. [The term xe2x80x9c(sub)harmonicxe2x80x9d, as used herein, means harmonic and/or subharmonic signal components.] Imaging of harmonic signals has been performed by transmitting a narrow-band signal at frequency f0 and receiving at a band centered at frequency 2f0 (second harmonic) followed by receive signal processing.
The technique of harmonic imaging using contrast agents is known. For example, harmonic imaging using contrast agents is disclosed by de Jong et al. in xe2x80x9cPrinciples and Recent Developments in Ultrasound Contrast Agents,xe2x80x9d Ultrasonics, Vol. 29, pp. 324-330 (1991). Contrast agent-generated harmonic imaging is capable of greatly improving image quality in vascular studies.
Contrast agents are typically encapsulated gas microbubbles between 0.1 and 10 microns in diameter. When introduced into the body by injection, contrast agents serve as high-reflectivity markers for blood flow and perfusion. Ultrasonic signals reflected from contrast microbubbles contain subharmonic components centered at half the transmit (fundamental) frequency as well as harmonic components centered at multiples of the transmit frequency. Isolating the (sub) harmonic signals for imaging is attractive because the contrast-to-tissue ratio is much higher for (sub) harmonic signals than for the fundamental signal.
A basic problem with (sub) harmonic imaging is how to separate the (sub) harmonic signal from the fundamental since the fundamental is usually much stronger than the (sub) harmonic signal. A pulse waveform for second harmonic imaging was described in U.S. Pat. No. 5,833,614. However, there is a need for pulse waveforms which have been optimized for subharmonic imaging.
In a preferred embodiment of the invention, a family of transmit sequences when converted into pulse waveforms, excite contrast agent microbubbles injected into the anatomy such that the ultrasonic subharmonic signal may be easily isolated for imaging. Transmit sequences that produce pulse waveforms having very low spectral energy at the subharmonic frequencies permit the subharmonic signal generated by the contrast agents to be extracted simply by filtering. Subharmonic imaging is thus improved by reducing the contribution of transmitted fundamental signals within the subharmonic band. This reduction is accomplished by transmitting a pulse derived from a transmit sequence belonging to a particular family of transmit sequences, and filtering the received signal to isolate the subharmonic signal for imaging. The resulting transmitted waveforms have low spectral energy within a band of frequencies centered at f0/2 and high spectral energy within another band of frequencies centered at f0, where both bands are within the transducer passband. The transmitted signal at frequency f0 is the fundamental frequency such that the contrastgenerated subharmonic signal may be extracted by a receive filter centered at frequency f0.
In accordance with a preferred embodiment, the pulse waveforms are bipolar (consisting of pulses of opposite phase corresponding to +1 and xe2x88x921 code elements of a transmit code sequence) and are constructed as follows: (1) start with L unipolar impulses, (2) repeat the L impulses M times, alternating the sign each time, and (3) repeat the entire sequence N times, where both M and N are greater than 1, and M is odd. The number L is inversely proportional to the transmit frequency.